This project explores protein-RNA interactions which are at the heart of the genetic code. Aminoacyl transfer RNA synthetases are enzymes found in all life forms and which are thought to be among the earliest proteins to appear. They catalyze attachment of amino acids to transfer RNAs which harbor the trinucleotides of the code. In this work, genetic approaches are used to understand the high specificity of the synthetase-tRNA interactions on which the code is dependent. The systems under investigation include bacterial, yeast and human enzymes and tRNAs. Specific projects include genetic manipulations to convert one synthetase into another, genetic engineering to enable a synthetase to switch its species specificity, manipulation of a synthetase to achieve a subtle change in the way that it recognizes atoms in the RNA minor groove, and determination of the specialized RNA recognition features of a human mitochondrial tRNA synthetase. Understanding of protein-RNA interactions in this system is broadly useful for understanding protein-RNA interactions in general. The disruption and alternation of specific RNA- protein interactions is a possible basis for developing therapeutic agents for treatment of human diseases.